In the art of semiconductor transistor structures, operating switching speeds of faster than about 20 picoseconds do not appear attainable either with conventional bipolar NPN (or PNP) transistor structures or with conventional unipolar N-MOS (or P-MOS) structures, even if their sizes be reduced still further, owing to natural. limitations of such structures that seem fairly well understood by workers in the art. Accordingly, these workers have been seeking to devise new transistor structures that are not subject to such limitations, in order to achieve picosecond transistor switching operation.
For example, in a paper by N. Yokoyama et al. entitled "Tunneling Hot Electron Transistor Using GaAs/AlGaAs Heterojunctions," published in Japanese Journal of Applied Physics, Vol. 23, No. 5, pp. L311-L312 (1984), a transistor structure is taught which promises to achieve picosecond or even subpicosecond operation. That structure relies upon a relatively thick 1,000 Angstrom (100 nm) base layer composed of n-type semiconductive gallium arsenide--that is, gallium arsenide doped with excess significant donor impurities to render it conductive, particularly in the transverse directions to the high-current path. The base layer is located between an emitter layer and a collector layer--each also composed of n-type gallium arsenide--a base-emitter barrier layer composed of semiconductive undoped aluminum gallium arsenide intervening between the base and emitter layers, and an undoped base-collector barrier layer (also composed of aluminum gallium arsenide) intervening between the base and collector layers. Each such barrier layer produces a potential (electrical and/or chemical) barrier against transport of electrons therethrough. Operation depends upon tunneling of electrons from the emitter to the base layer in response to a positive voltage applied to the base layer with respect to the emitter layer. When this voltage is high enough, electrons will enter the base layer with a sufficient kinetic energy ("hot electrons") to pass through the base and be collected by the collector layer--provided that these electrons, after passing through the base layer where scattering and/or trapping reduce their kinetic energies, still have sufficient energy to surmount the base-collector barrier.
Such a transistor structure, however, suffers from a relatively low value of .alpha.=.beta./(1+.beta.) where .beta. is the current gain of the transistor (i.e., the ratio of the increment of collector current to an increment of base current) and .alpha. represents the "transfer ratio"--i.e, the fraction of electrons injected (as by tunneling or other phenomena) from the emitter into the base that are collected by the collector. Specifically, the transistor had an undesirably low value of transfer ratio .alpha. of only about 0.28, whereas a desirable value of .alpha. would be at least about 0.50 and preferably much more. It is believed that this undesirably low value of .alpha. is caused by the relatively thick base layer in which the electrons passing therethrough, going from emitter to collector, lose a relatively high fraction (or even all) of their kinetic energy because of the scattering phenomena and/or trapping in the base and hence cannot then surmount the base-collector barrier. On the other hand, reducing the thickness of the base layer to reduce this loss of kinetic energy therein would result in an undesirably high transverse or "spreading" base resistance--which would increase the RC base delay, and hence would undesirably increase the transistor switching time to values equal to or greater than those of conventional present-day transistors. Accordingly, it would be desirable to have a transistor structure capable of picosecond operation and having a transfer ratio .alpha. of at least about 0.50.